Federal Regulations effective in 1992 require curb ramps to be provided with detectable warning in the form of raised truncated domes with a nominal diameter of 0.9 inches or 23 millimeters, a nominal height of 0.2 inches or 5 millimeters and a nominal center-to-center spacing of 2.35 inches or 60 millimeters. The truncated domes are required to visually contrast with the adjoining surface by assuming light-on-dark or dark-on-light contrast patterns. Furthermore, the material used in producing the contrast is required to be an integral part of the structure's surface.
The purpose of such ramp constructions is to alert users to their presence by providing tactile as well as visual indications or warnings. The presence of truncated domes provides a tactile warning detectable by foot contact, contact by wheels including wheelchair wheels and bicycle wheels and by contact with walking aids such as canes. The tactile warning is particularly functional in alerting the visually impaired or blind to the presence of ramps.
Furthermore, the contrasting visual pattern provided between the truncated domes and the surrounding ramp surface is particularly suited for alerting those users who are not visually handicapped as to the presence of such ramps. This contrasting visual scheme is readily discernible to oncoming traffic, be it foot traffic or more rapidly approaching vehicular traffic, such as bicycle traffic.